1. Field of the Invention
The present invention relates generally to a strip joining apparatus for metal pipe mill equipment which cuts a trailing end of a preceding steel strip and a leading end of a following steel strip that are to be supplied to a looper of the pipe mill equipment, and puts the cut surfaces of the two steel strips into contact with each other, and then welds them to each other, thus joining the two steel strips and, more particularly, to a strip joining apparatus which can effectively weld and join comparatively thick strips having thicknesses ranging from 15 mm to 25 mm to each other.
2. Description of the Related Art
Generally, a steel strip used to produce a metal pipe is supplied to a looper in such a way that a roll formed by winding a strip by a predetermined length (from 50 M to 100 M) is mounted on an uncoiler, and the strip is supplied from the uncoiler to the looper. The steel strip is thereafter transferred from the looper to a pipe producing machine, that is, a pipe mill, for producing a metal pipe.
The looper rotates and winds the steel strip around it, thus storing a predetermined length of steel strip, and simultaneously supplies the steel strip that has been stored to the pipe mill. As such, the looper stores a predetermined amount of steel strip and supplies the steel strip to the pipe mill. Such loopers are classified into a vertical type looper and a rotary type looper that is an improvement of the vertical type looper. The detailed construction and function of loopers are well known.
Typically, a steel strip must be continuously supplied to a pipe mill process. Thus, if after a steel strip roll has been completely uncoiled, another steel strip roll is mounted on the uncoiler and supplied to the pipe mill.
Here, a trailing end of the steel strip that has been completely uncoiled must be welded to a leading end of the steel strip that has been newly mounted on the uncoiler, thus joining them to each other, thereby forming a continuous steel strip. During this joining operation, the transfer of the steel strip is interrupted, but the steel strip that has been stored in the looper is supplied to the pipe mill to prevent the joining operation from interrupting the pipe mill.
In the conventional art, the welding operation has been typically carried out using a manual method. However, in this case, the welding operation cannot be rapidly carried out. If the operation of joining the steel strips to each other is not finished before the steel strip that has been stored in the looper is completely exhausted, this results in the problem of the entire pipe mill being interrupted.
To solve the above problem that the manual work causes, there is conventionally available a welding device (Apparatus for cutting and welding strips) as disclosed in Korean Patent Registration No. 10-649652 (registered on Nov. 17, 2006), wherein a trailing end of a preceding steel strip and a leading end of a following steel strip that is supplied to a looper are cut, and then the cut surfaces of the two steel strips are put into contact with each other and are welded to each other (hereinafter, referred to as the ‘conventional technique’). As shown in the front view of FIG. 1A and the plan view of FIG. 1B, the conventional technique includes clamp units 20 and 22 which respectively press adjacent ends of steel strips 10 and 11 and clamp them, a cutting unit 30 which cuts the steel strips 10 and 11, a transfer unit 40 which transfers the clamp units 20 and 22 so that the cut surfaces of the two steel strips 10 and 11 are put into contact with each other, and a welding unit 50 which welds the steel strips 10 and 11 to each other. The terms of the elements designated by reference numerals of FIGS. 1A and 1B are the same as those stated in the patent gazette of the conventional technique.
The conventional technique having the above-mentioned construction is a strip joining apparatus which can be used to cut comparatively thin steel strips and weld them to each other, but it is not suitable for being used to join together thick steel strips which range in thickness from 15 mm to 25 mm.
In detail, in the strip joining apparatus according to the conventional technique, a cutting blade of the cutting unit 30 moves downwards perpendicular to a steel strip, thus cutting the steel strip. Hence, as shown in FIG. 2A, the cut surface of each of two steel strips runs vertically. If the vertical cut surfaces of the two steel strips are put into contact with each other and are welded to each other, the maximum thickness of the steel strips which can be welded and joined to each other without creating any problems ranges from about 10 mm to about 12 mm.
However, in the case of comparatively thick strips, if the cut surfaces of the strips are vertical surfaces, as shown in FIG. 2B, only upper portions of the two cut surfaces of a preceding strip and a following strip are welded to each other, while the space between their lower portions is not filled with deposit metal, because a portion between the two cut surfaces that requires welding is comparatively deep.
Particularly, if the quality of welding of the joined portion between the steel strips is low, during a process of twisting the steel strips in the looper or of transferring it to pipe mill equipment, there is the possibility that the steel strip will be cut on the joined portion, or that the manufactured metal pipe will be defective. To mitigate the above problems, a worker may separately manually weld the lower portions of the steel strips. However, the manual work makes it difficult for the quality of welding to be uniform and is time-consuming.
In an effort to overcome the problems of the conventional technique, especially, to enhance the quality of welding of thick strips, a process of cutting steel strips and welding the strips to each other before the joined strip is supplied to the pipe mill equipment may be conducted using the method of: trimming the cut surfaces of the preceding steel strip and the following steel strip using a milling cutter or the like so that the cut surfaces of the two steel strips that are put into contact with each other define a ‘V’-shaped groove, as shown in FIG. 2C; and welding the cut surfaces along the ‘V’-shaped groove.
However, given the characteristic of the pipe mill that the process of cutting the steel strips and welding them to each other must be completed within a time period of from 8 minutes to 15 minutes, the trimming of the vertical cut surfaces of the thick strips using a cutting tool, to form a ‘V’-shaped welding groove, makes the strip joining work take a lot of time, although it can enhance the quality of welding at the junction between the thick strips. If the storage capacity of a looper for a steel strip is not large enough, the strip joining work may be pressed for time, because it must be finished within a relatively short time (before an extra steel strip that has been stored in the looper is completely exhausted).
Particularly, the pipe mill equipment forms a pipe while a steel strip is being supplied thereto at high speed of about 50 m per a minute. Given that the strip joining work must be finished before the extra steel strip that has been stored in the looper is completely exhausted, an improved strip joining apparatus which can effectively conduct the process of cutting thick steel strips and welding them to each other in a comparatively short time is required.